The neurophysiology of behavior is being studied in an Aplysia preparation developed in this laboratory. We have identified neurons which control parameters of habituation, i.e., response decrement and dishabituation. Habituation is controlled by a hierarchically organized pathway involving the central and peripheral nervous systems, with the central nervous system exerting its action via the peripheral nervous system. Central control is dominant, is more adaptable to changes in stimulus conditions, and is age-dependent, i.e., absent in young animals, thus development of control and its neural substrates can be studied. Habituation, considered an archetype of learning, involves comparatively simple neural networks in Aplysia, and thus its physiological correlates are amenable to analysis. The neural pathway revealed by our work should aid in achieving the following objectives: 1) to investigate the physiological basis of greater adaptability of control neurons as compared with peripheral neurons, using the habituation paradigm; the comparison can reveal properties characteristic of central neurons. 2) to investigate the hierarchical relationship between central and peripheral neurons controlling habituation. The neural circuits in the habituation pathway may be prototypic of those controlling more complex behaviors. 2) to examine electrophysiological and morphological properties of central neurons, involved in habituation, of young and older animals. The development of control and of properties characteristic of central neurons can be studied. 4) to study changes in synaptic efficacy during habituation. Our findings indicate that inhibition is responsible for control of habituation. 5) to correlate neural coding with parameters of habituation; the parameters appear to be dependent upon spike frequency in single neurons.